1. Field of the Invention
The present invention relates to measuring aspects of a liquid by determining the capacitance of a volume of liquid. More particularly, the present invention relates to using capacitance in a dispensing or aspirating probe, particularly in automated analyzers, such as chemistry or blood analyzers, to determine the volume of liquid in the probe tip or the presence of air bubbles in the liquid in the probe tip.
2. Description of the Related Art
Advances in industries utilizing chemical and biological processes have created a need for the ability to accurately and automatically dispense varying quantities of liquids for commercial, laboratory, and research uses, particularly in automated analyzers. Typically, an automated analyzer includes an automated liquid metering system that automatically aspirates a sample of liquid from a reservoir or test-tube containing patient sample and dispenses the liquid into another vessel, such as a reaction cuvette. The liquid moving system typically includes a pipette or probe that accomplishes the aspirate and dispensing functions under the control of a robotic arm.
Automated analyzers are widely employed in blood bank centers, hospitals, and clinical laboratories. Tests utilizing chemical reagents specific to the test being conducted (i.e. HCV, HIV) are dispensed into another container, e.g., a reaction chamber containing the sample to be tested (serum, plasma, blood). The resulting reaction products that arise are subsequently analyzed (i.e. luminescence, optical properties, electrical properties) and results are generated. Typically the results are stored electronically and printed out for review.
Monitoring and controlling aspiration or dispensing of a liquid is known in the art. Several methods and techniques exist for doing the same. For example, U.S. Pat. No. 6,158,269 uses a pressure based technique. Pressure is monitored (air or fluid column) using a pressure transducer and the data collected are then quantified and used to make determinations as to the state of fluid. U.S. Pat. No. 4,794,085 discloses using a vacuum to determine when a probe is approaching and has contacted a liquid to be aspirated.
Other methods include optical measurement systems which quantify the optical index change characteristics of a liquid system to verify when liquid has been dispensed. Other known optical approaches entail using an optical path (beam) which when broken corresponds to liquid being dispensed.
Other publications for monitoring and controlling aspirating or dispensing include U.S. Pat. Nos. 6,250,130, 5,450,743, 6,121,049, 5,443,791, 6,317,696, 5,045,286 and EP Patent Application No. 1209471 A2.
The use of capacitance and other methods to measure an aspect of a liquid is also known in the art. Liquid level methods are used to sense the liquid level in a container and then determine the quantity of liquid in the container based on its geometry. As stated above, liquid level methods are implemented using various technologies (capacitive, optical, pressure). Examples of capacitive determination of liquid levels includes WO 98/57132 and EP 1091198 A1. Other uses of capacitance are described in U.S. Pat. No. 4,897,244 which discloses the use of capacitance to determine when a probe contacts a liquid.
Much of the known art only controls the proximity of the metering probe to the surface of the liquid to be aspirated. These methods and techniques, however, do not measure the actual volume of liquid that is aspirated or dispensed. This is generally done indirectly. That is, many known aspirating and dispensing probes uses a syringe pump coupled to a stepper motor to aspirate or dispense a liquid. The volume of liquid aspirated or dispense is determined by the number of steps performed by the stepper motor. This will generally result in an accurate determination of liquid volume as long as what is being dispensed or aspirated is truly liquid. Problems can arise if, for example, the probe is not fully immersed in the liquid or if there are air bubble(s) entrained in the liquid. Both of these conditions can result in air volume being read as liquid volume, thus leading to less liquid being aspirated or dispensed than was intended. Accurate metering of liquid is important in applications such as diagnostic analyzers or blood analyzers.
Other problems associated with much of the known art include lack of robustness of the device used to monitor and control aspiration and dispense. That is, many of the known systems are susceptible to mechanical vibrations and other perturbations. Yet other problems lie in the size of the systems to monitor aspiration and dispense in that known systems often require components away from the aspirating probe and probe tip. For example, U.S. Pat. Nos. 6,158,269 and 6,484,556 require a pressure transducer between the stepping motor and the probe. For the foregoing reasons, there is a need for increased improvement in the measurement of liquid being aspirated or dispensed. There is also a need for an increased improvement in the robustness or reliability of a system or apparatus to make such measurements and in consolidating the components for such measurement in a central location, preferably at the point the measurement is taken.